Neumann V475 -- passing signal but with very low output

[Shattersignal]

Hi there!  I just wired up my Neumann V475-2c.  I'm planning on using it with just one left and one right input for now--to add some coloration where it might be needed on a master bus. 

I wired the + and - sides of my inputs through 10k resistors (RE) and I left the gain-selecting pins open (RG), which ought to bring the output to nearly unity.  I also terminated the outputs with 600ohm resistors, since I'm running the unit into my Lavry AD.  The module is getting 24vdc and 0v from my PSU. 

The problem is that I'm seeing an extremely low (-50dB?) output from the module.  Could it be that some ICs are faulty?  Is there an obvious explanation for this low output?  Both sides seem to be displaying identical symptoms. 

Thanks a ton, if you have any ideas that might help me to troubleshoot!  I can't wait to put that awesome German iron into service.

 

[Humner]

Hmm, looking at this schematic

http://www.gyraf.dk/schematics/Neu_v475.gif

Would you not need to jumper pins 12 and 13 together and then jumper pins 18 and 19 together? Or at least provide a suitable gain resistor here to give you the level you require?

 

[[silent:arts]]

For unity gain
RE = (2x) 5K11 and RG = 6K19

RG is specified 0 to 15K, leaving it open is a bit high ....

 

[Shattersignal]

Thanks guys.  You're both right, but I don't think that's the problem here.  My  RE is 2 x 10k, which should arrive at near unity with open RG, based on what I've read.  You don't necessarily need to use 5k1 RE, from what I understand--the unit was designed to provide differing headroom options depending on the number of inputs you use and there is more than one ratio of RE and RG that get you to the same gain.  The fact that I'm using just a single input on each side shouldn't matter either, from what I know about active summing--but please correct me if I'm wrong about any of this.  I used 10k RE to save the trouble of needing to hook up the RG and because I have a few 10k resistors laying around. 

Regardless, I'm seeing about 40 or 50 dB of loss between the input and output and I really can't figure out why that would be the case.  I'm using 8a, 8c, and 26a, 26c for my inputs.  The only wiring-related problem that occurs to me is that I haven't wired up any grounding scheme, except for the 0vdc rail from the psu.  With a zero-ohm input, there's no reason that I'd need to wire an audio star ground for the zero ohm "summing network" to work, is there?  The amp doens't care about my audio ground, right?  Right now, my input and output shields/drains are dangling.  I figured I'd bread-board it before worrying about the audio ground scheme, case drilling, etc. 

I'm seeing the same level drop on both channels, so I'd be surprised that failing parts were to blame, although I suppose it's a possibility.  Do you guys have any ideas?  The electrolytics *look* good.  Are those ICs suceptible to static?  This card has been passed around quite a bit before it fell into my hands. 

 

[Shattersignal]

I'll throw some 15k RGs on there, I guess.  Do I need two per channel?  2a to 2c and also 4a to 4c (and likewise for 30a/c and 32a/c)?

Update #1: I just tried the 15k RGs--no solution.  Now I'm going to go ahead an find some 5k1 resistors to replace my 10k resistors for the RE, as specified on that datasheet.

Update #2: tried (2x) 5k1 RE--no solution.  I'm beginning to think I'm not using the correct pins.  Can anyone clarify the proper pinout for the V475-2c?  I haven't actually seen a schemo for the 2c model, so perhaps I'm not using the correct input and RG pins.  I traced one of the input transformer coils to the pin header, but perhaps I'm on the wrong coil?

 

[[silent:arts]]

You know edited to the C version, which is different to what I posted above (for the A model)

 

[Shattersignal]

[silent:arts] does this mean that for an RE of 5k1 I need an RG of 10k?  I'm not sure how to interpret this.  I don't think that setting my RG is what's at issue, actually.  I'm not really concerned about a dB or two when my level is off by 40dB. 

Do you know the pinout for the 2c model?  I suspect that I am connected to the wrong pins.

Thanks a ton for the info.  When my card is confirmed working, I'm sure I'll be interested to dial in the gain perfectly.

 

[[silent:arts]]

2 x 5K11 RE and 2 x 13K RG = unity gain

schematic with pinout:
http://groupdiy.com/index.php?topic=44805.msg561359#msg561359

 

[Shattersignal]

Thanks very much.  Yes, I have used the correct pins.  I suppose I should replace my ICs?

Update: I went ahead and ordered new ICs.  I hope that fixes it, because I really don't have a clue why it isn't working correctly right now.

Update: I installed all new ICs on one channel.  The problem persists and operation is identical to the other channel.  Am I correctly wiring the front end?  My differential audio signal has a 6.2kohm resistor in series on both the + and the -.  Is there any other wiring that's necessary?  I'm fairly sure I'm on the correct pins.  Can anyone who has a V475-2c model please confirm the pinout for me?  Each channel needs only VDC+, and power ground, differential audio input + and -, differential audio output + and -, and an RE resistor jumper.  Is there something obvious that I'm missing?  Thanks a ton!

 

[Shattersignal]

Any Neumann V475 users/builders out there that have a guess about what's wrong with my card (or my wiring)?  I'm totally out of ideas at this point.  Thanks!

 

[moamps]

Do you have a DMM? Could you post some pictures of your module; component side and pcb side, and wireing you did so far?

 

[Shattersignal]

Will do.  And of course I have a DMM   

Thanks, MoAmps.

 

[Shattersignal]

So first off, let me give an overview of what I've done.

I believe that my unit is a V475-2C (although it says both 2B and 2C at different places on the unit).  I don't have a pinout for a 2C (I've scoured the earth for one, to no avail), so I traced out one of the coils from each of the input transformers to the pins and I assume that this is the differential audio inputs--pins 8a, 8c, and 26a, 26c.  I'm currently using 5k1 resistors for the RE value in series with the + and - balanced audio (I have also used 10k, as stated earlier in the thread).  I connected the outputs to 12a, 12c, and 22a, 22c, and terminated them at 600 ohms.  I have delivered 24vdc+ to 16a and 18a, and 0v to 16c and 18c.  I have made jumpers with 15k resistors for the RG positions.  Again, I have no pinout, so I have experimented with different pins.  I think they seem to have an effect on level when strapped across 2a/4a, and 30a/32a.  I have tried many positions, however.

The amp passes signal, but the output is extremely low--maybe around 40dB or 50dB down. 

I even ordered new ICs for one of the two channels, and operation is the same. 

Am I not using the correct pins?

Here are photos of my unit and wiring:

https://www.dropbox.com/sh/r39qlpx7uw82gir/AACpr3w1YsWgDW22FhMOhohua?dl=0

Thanks a ton, MoAmps (and anyone who has insight into wiring these units)!

 

[moamps]

That's 475_2c and the schematic silent_arts already sent is here:
http://groupdiy.com/index.php?topic=44805.msg561359#msg561359
We will referring to it so please download it.

First, could you confirm DC voltages according to picture below?

 

[moamps]

And, are you sure that +24V is correctly connected? Your red wire goes to "C" and black to "A" line.

 

[Shattersignal]

MoAmps, I was just exploring potential problems in the PSU when you typed this.  After reading, I swapped the rails and now the test points check out!

I'll put a signal on the input and see if the problem persists. 

Thanks! 

I'll post results asap.

 

[Shattersignal]

Hahaha it works!

*facepalm*

Man, thanks for all the wonderful assistance, MoAmps, and also Silent Arts. 

Sheesh.  I though I was doing alright translating those German schematics.  I guess I made one simple mistake--albeit a rather serious one!

Seriously, I am so extremely grateful.

Now I get to enjoy this marvel of German engineering.  Also, I have to say, I'm additionally impressed with the engineering after hooking up the DCV backwards and finding that the unit still works   

Group DIY for the win!!! 

 

[synthiaks]

Also, I have to say, I'm additionally impressed with the engineering after hooking up the DCV backwards and finding that the unit still works   

Thank the T1 and  D1 ! The diode protect against any Wrong Bias and doesn't turn on T1 if it gets - where + should be. So with wrong polarity  T1 transistor won't feed any Current in the circuit.